Hydrogen and deuterium-filled ultraviolet discharge lamps ("UV lamps") are well known. Examples are described in U.S. Pat. Nos. 5,159,236 (INDIRECTLY HEATED CATHODE FOR A GAS DISCHARGE TUBE) and 4,910,431 (HYDROGEN DISCHARGE UV LIGHT SOURCE OR LAMP, AND METHOD OF ITS MANUFACTURE), the disclosures of which are hereby incorporated by reference. UV lamps provide a broad-band continuous spectrum ultraviolet light, (generally in a range between the wavelengths of approximately 160 nm and approximately 360 nm. UV lamps are used in applications such as high performance liquid chromatography (HPLC), ultra violet visible absorption spectroscopy (the measurement of the wavelength and intensity of absorption of near-ultraviolet light by a sample), and atomic absorption spectroscopy (AA).
Typically, a UV lamp comprises a sealed quartz or ultraviolet glass bulb filled with a gas, such as hydrogen or deuterium. It is also known to use neon, argon, krypton and xenon in addition to H.sub.2 /D.sub.2 bulbs. The bulb also contains an anode/cathode combination. The cathode is traditionally a double-coil tungsten wire. "Double-coil" describes a wire comprising a primary and secondary coil. That is, a length of wire has been wrapped around a mandrel to form a primary coil, and the primary coil is then wrapped around a second, larger mandrel to form a secondary coil. The cathode is coated with an electron-emitting material, such as an alkaline earth oxide (e.g., barium oxide, calcium oxide, strontium oxide,) etc. or an alkaline earth mixture.
UV lamp cathodes can either be directly heated ("hot cathode") or indirectly heated (see '236 patent, supra). Heating the cathode raises the temperature of the emitter material until it freely emits electrons, or becomes thermionic. When an appropriate voltage is applied across the anode and cathode, and the cathode is thermionic, the cathode will emit electrons allowing a current flow between cathode and anode. The electrons of the current flow collide with the hydrogen or deuterium gas and enhance the conduction path by forming ions. In the hot gas, molecules of hydrogen (or deuterium) are excited and emit continuous ultraviolet radiation by decay to a lower state.
An unavoidable characteristic of thermionic electron emission is the sputtering away of the emitter material. As the emitter material decreases, the cathode voltage drop increases. Accordingly, a UV lamp design must balance the need to emit electrons with the need to conserve emitter material to ensure a long, useful life.
Currently, both 10 V and 3 V filaments are used in UV lamps. That is, a 10 V potential is impressed on the filament to generate the desired cathode temperature. It is also known to apply coatings of electron-emitter materials to filaments. One example is U.S. Pat. No. 2,306,925 (ELECTRODE AND ITS FABRICATION), the disclosure of which is hereby incorporated by reference. Emitter material is applied to a 3 V cathode by dipping the cathode into a triple carbonate solution (e.g., barium carbonate, calcium carbonate and strontium carbonate). The carbonate solution is allowed to dry onto the cathode. It is then oxidized to create a more stable alkaline crystal oxide (e.g., barium oxide, calcium oxide and strontium oxide). Dipping applies a layer of emitter material to the cathode of a 3 V UV lamp adequate to provide free electrons and acceptable product life before substantial evaporation.
However, dipping of known 10 V cathodes, in many cases, is not possible. The dimensions of cathode wire are driven by electrical requirements (e.g., attaining 10 V potential) and thermal requirements (i.e., the cathode must reach the thermionic temperature of the emitter material). The 10 V cathodes are normally made of wire approximately half the diameter of the 3 V cathode wire. The thinner wire used for a 10 V cathode is less rigid than that of a 3 V cathode, and has much smaller primary and secondary coils. These two factors make application of the emitter material by dipping impractical. Dipping a double-coil 10 V cathode will completely fill the interstices of the primary and secondary coils with emitter material. Thereafter, movement of the pliable double-coil 10 V cathode is likely to loosen and flake away the crystalline emitter material filling the interstices of the double-coil. Thus, the effective life of the double coil 10 V filament is substantially diminished. It is known to use a spraying technique to apply emitter material to 10 V cathodes. However, spraying is more expensive than dipping, and it applies less emitter material. Thus, the material evaporates more quickly, sometimes making the life of the UV lamp unacceptably short (i.e., less than 1000 hours).
Accordingly, it is desirable to provide a ultraviolet discharge lamp using the industry-standard 10 V cathode also having a useful life greater than 1000 hours.